The coupling reaction of pinacolborane with various aryl bromides in the presence of a catalytic amount of Pd(OAc)(2) together with DPEphos as ligand and Et(3)N as base provided arylboronates. High yields were obtained in the case of electron-donor substituted aryl bromides. The direct preparation of arylboronates allowed the one-pot, two-step synthesis of unsymmetrical biaryls in high yields. [reaction: see text]
[Structure: see text] Direct C-H arylation of purines to position 8 by diverse aryl iodides was achieved with Pd catalysis in the presence of CuI and Cs2CO3. The methodology is general and efficient and was applied in the consecutive regioselective synthesis of 2,6,8-trisubstituted purines bearing three different C-substituents in combination with two cross-coupling reactions.
A new regioselective synthesis of 5- and 6-aryluracil bases based on direct C-H arylations of diverse 1,3-protected uracils has been developed. Benzyl-protected uracils were selected as the most practical in terms of stability during the arylation and facile cleavage of the benzyl groups. Pd-catalyzed C-H arylations in the absence of CuI gave preferentially 5-aryl-, whereas the reactions in the presence of CuI gave 6-aryl-1,3-dibenzyluracils. Final deprotection either by transfer hydrogenolysis over Pd/C or by treatment with BBr(3) gave the desired free arylated uracil bases in good yields.
Pd-catalyzed direct C-H arylation of unprotected purine nucleosides with aryl iodides at position 8 was developed to allow a straightforward single-step introduction of diverse aryl groups.
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